1. Technical Field
The present invention relates to an optical filter, an optical filter module, a spectrometric measurement apparatus, an optical apparatus, and other apparatuses.
2. Related Art
An interference filter has been used in a spectrometric measurement apparatus and other optical apparatuses. A known aspect of the interference filter selects a wavelength of light that passes therethrough (Fabry-Perot etalon interference filter) (see JP-A-11-142752, for example). A transmissive variable wavelength interference filter described in JP-A-11-142752 (hereinafter sometimes referred to as a variable gap etalon filter or simply referred to as a variable gap etalon), which can select the wavelength of light passing therethrough, includes a pair of substrates held in parallel to each other and a pair of multilayer films (optical films) formed on the pair of substrates so that the multilayer films (optical films) face each other and have a fixed-distance gap therebetween. The interference filter selects the wavelength of light passing therethrough by using an external force to change the size of the gap between the pair of multilayer films (optical films).
When a single variable wavelength filter (variable gap etalon filter, for example) is used to cover a desired wavelength band and the transmitted light has a wide wavelength range, an actuator is required to move a movable portion of the variable wavelength filter over a long distance. The moving of the movable portion disadvantageously increases the moving range of the movable portion of the variable wavelength filter and hence increases the voltage for driving the movable portion.
Another problem with a wide wavelength range of the transmitted light is that the structure of the optical films for providing a necessary band of the variable wavelength filter is complicated. In other words, the optical films need to be designed so that the entire desired wavelength band can be covered, resulting in a complicated structure of the optical films. When the structure of the optical films is complicated, it increases a burden related to manufacturing processes and decreases the intensity of the transmitted light. Other inconveniences also tend to occur.